Inkjet hardcopy devices, and thermal inkjet hardcopy devices such as printers, plotters, facsimile machines, copiers, and all-in-one devices which incorporate one or more of these functions in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted onto the print media such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye. Inkjet printers may use a number of different ink colors. One or more printheads (also sometimes referred to as "pens") may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer frequently can accommodate two to four print cartridges. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the media which is to be printed. Each print cartridge typically has an arrangement of individually controllable printhead ink ejection elements for controllably ejecting the ink onto the print media, and thus a certain width of the media corresponding to the layout of the ink ejection elements on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
Each ink ejection element, or firing unit, includes an ink chamber connected to a common ink source, and to an ink outlet nozzle. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet printers, the transducers are thin film firing resistors that generate sufficient heat during application of a brief voltage pulse to vaporize a quantity of ink sufficient to expel a liquid droplet.
A power source supplies electrical power (a certain amount of current at a certain voltage) to the firing resistors in the ink ejection elements in order to provide the electrical energy required to fire ink drops from the elements. The energy applied to a firing resistor affects performance, durability, and efficiency. It is well known that the firing energy must be above a certain threshold to cause a vapor bubble large enough to expel a drop to nucleate. Above this threshold is a transitional range, in which increasing the energy increases the drop volume expelled. Above a higher threshold at the upper limit of the transitional range, drop volumes do not increase with increasing energy. It is in this upper range in which drop volumes are stable even with moderate energy variations that printing ideally takes place, because the variations in drop volume that cause disuniformities in printed output can be avoided when operating in the upper range. As energy levels increase above this optimal zone, uniformity is not compromised, but rather energy is wasted resulting in excessive temperature rise, and the printer components are prematurely aged due to excessive heating and ink residue build up.
In existing systems having a dedicated connection for each firing resistor, a one time calibration of each connection by printer or production circuitry external to the pen also compensates for any parasitic resistance or impedance in the unique path leading to each resistor. Printheads may be characterized at production to set these operating parameters. In addition, because each interconnection pad was only required to carry enough current to fire a single resistor, the area of the pad needs to be only large enough to support a single contact point with the printer electronics.
However, in highly multiplexed print heads having different sets or groups of nozzles, each set addressed by a common voltage line, there may be variations due to other factors. Each set or group of nozzles is powered by a single voltage line that receives power via an electrical contact pad between the printer electronics and the removable print cartridge. This line continues on a flex circuit to a tab bonding connection to the printhead die having other electronics, including the firing resistors. The impedance of the print cartridge contact pads, tab bonding connections, and the flex circuit trace connections in between can vary from cartridge to cartridge, from nozzle to nozzle, and over time for a given cartridge, even when the voltage provided by the printer to each of the cartridge contacts is well controlled. Consequently, as printed data changes, the current drawn through the line and the voltage as measured at the print cartridge terminals may be undesirably varied. For instance, when many or all nozzles are fired simultaneously, the voltage may be depressed by parasitic effects, giving a lower firing energy than when only one or a few nozzles are fired. In the past, however, the power and ground interconnect pads were intermixed with the logic signal pads without sufficient consideration of the voltage variances that might result during printing. Accordingly, there is still a need for a thermal inkjet printer using these new, highly multiplexed printheads that effectively compensates for the voltage variations due to parasitic resistance in order to provide uniform ink drop volumes yielding printed output of high quality.
In a preferred embodiment, the present invention provides a multipass inkjet printing system having modulated firing pulses that produce uniform ink drop volumes from print cartridges which have a large number of ink ejection elements. The die which includes the ink ejection elements has multiple power bond pads for receiving the power signals which control the operation of the ink ejection elements. A flex circuit has power interconnect pads electrically connected to power tab leads which in turn are electrically connected to the power bond pads of the die. A power source detachably connected to the power interconnect pads supplies the power signals to the printhead die. Energy management circuitry adjusts the firing energy delivered to the ink ejection elements to produce the desired uniform drop volumes.
Typically, the ink ejection elements are organized into groups, with at least some of the ink ejection elements in each group selectively actuated by a different one of the firing pulses. A preferred embodiment has four groups and four independently-controlled firing pulses from four independent energy management circuits. The flex circuit may also have one or more ground interconnect pads electrically connected to ground tab leads which in turn are electrically connected to the ground interconnect of the die for carrying return current for the power signals. In order to minimize to parasitic resistances internal to the printing system, the power and ground interconnect pads are typically located in a region closer to the printhead die than other pads for logic signals. The electrical connections between the pads and the tab leads on the flex circuit preferentially are conductive traces. Typically the flex circuit is attached to a surface of the print cartridge. Each of the power and ground interconnect pads has at least one contact point through which current is supplied by the printer, but in order to supply amounts of current in excess of what a single contact point can accommodate, at least some of the pads have at least two contact points. Pads which have at least two contact points allow improved calibration of the printhead, since current can be supplying during calibration through one contact point, and the voltage drop due to the internal resistance of the flex circuit and the printhead die can be more accurately sensed through another contact point on the same pad. The power tab leads arc preferentially spaced around the periphery of the printhead die; in a preferred embodiment having a rectangular die and four ink ejection element groups, the four power tab leads are located at the corresponding four corners of the die, and two ground tab leads are located between the two power tab leads on the shorter sides of the die.
Another embodiment of the present invention includes an interconnect circuit for distributing the power signals to the ink ejection elements. The circuit has a flex circuit on which electrically conductive interconnect pads are fabricated, each pad for carrying one of power or ground signals, with electrically conductive elongated traces also fabricated on the flex circuit for connecting at a different location to the ink ejection elements. The flex circuit preferentially also contains an isolation trace (also sometimes referred to as a "squish" trace) located in between two of the power traces in order to electrically isolate the power signals carried on the power traces from each other. One or more punched holes injunction regions of at least two isolation traces isolate the isolation traces each others and from connection to any of the pads.
The present invention may also be implemented as a method of supplying power to a printhead having multiple ink ejection elements. The pulse widths of at least two firing pulses are independently modulated and transmitted to the ink ejection elements to controllably eject ink drops of uniform drop volume. Each firing pulse is preferentially connected to more than one ink ejection element, with only one firing pulse being connected to any individual element. Typically the ink ejection elements are divided into groups, with a separate firing pulse being connected to each group. The ink may be provided to the ink ejection elements from an on-carriage reservoir contained in a print cartridge housing the printhead, from an off-carriage reservoir fluidly connected to the printhead, or from an on-carriage reservoir removably attached to a print cartridge housing the printhead. The voltage level required for the firing pulses is determined during a calibration operation including measuring the internal resistances of the printhead.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.